Our listeners, who are among the smartest we know, jumped in to the comments to untangle the mystery. First LM (Zoetrope46) on the Mark Twain quote:

Please, correct your small mistake in the above article. :) You have misquoted/misattributed Mark Twain. Clarence Darrow said, "I've never killed a man, but I've read many an obituary with a great deal of satisfaction." Twain's quote is actually, "I didn't attend the funeral, but I sent a nice letter saying I approved of it." Thank you.

Then Courtland Park (copark) on the MLK quote:

Ryan Rooney's "mis-attributed" MLK quote can be found in Strength To Love by MLK on page 53. The book is on Google books. It's not exact, but I think we can agree that it was an MLK quote.

Zoetrope was right on. Copark was sort of, kind of right. The MLK quote that went viral online was, "I will not rejoice in the death of one, not even an enemy. Hate cannot drive out hate; only love can do that."

The Atlantic's Megan McArdle did some sleuthing, because that MLK quote sounded funny to her. "I assumed that someone had made it up on purpose," she writes. "I was wrong."

Turns out that the first person to use it was Facebook user Jessica Dovey; except she wrote a sentence of her own, followed by a quote from MLK. The original Facebook status was this:

I will mourn the loss of thousands of precious lives, but I will not rejoice in the death of one, not even an enemy. "Returning hate for hate multiplies hate, adding deeper darkness to a night already devoid of stars. Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate, only love can do that." MLK Jr.

Somewhere along the line, the status update was shortened and a quote mark was dropped and like a game of internet telephone a new, mangled quote was born.